The effects of pneumonia on the pharmacokinetics of chloramphenicol, lincomycin, and oxytetracycline were evaluated in two-month-old calves. Pneumonia was induced by injection of Pasteurella haemolytica cultures directly through the thoracic wall into each lung. Six days prior to induction of pneumonia, the antibiotics were administered in a single i.v. dose. The antibiotics were administered again 48 (i.v.), 60 and 72 h (i.m.) following injection of P. haemolytica. The pharmacokinetics of chloramphenicol (25 mg/kg) and lincomycin (10 mg/kg) were not significantly different in calves with pneumonia. The hybrid rate constant beta for oxytetracycline was increased in calves with pneumonia from 0.0034 +/- 0.0003/min to 0.0048 +/- 0.0007/min between 2 h and 8 h. Thus the elimination half-life in serum was shortened from 212.4 +/- 20.3 min to 149.3 +/- 19.5 min. In addition, there was an apparent but not statistically significant decrease in K12 with pneumonia. These findings accentuate the need for observance of 12-h dose intervals with oxytetracycline.

The pharmacokinetics of three antibiotics--gentamicin, neomycin and oxytetracycline were determined in newborn calves. The kinetic determinations, using two-compartment open models, were made at increasing ages from 1 day to 42 days and compared with those made from older calves (250+ days). Although all three antibiotics are eliminated unchanged primarily by glomerular filtration, there were marked differences in the development of elimination processes for individual drugs. The pharmacokinetics of neomycin were not influenced by age. Although the elimination half-life of gentamicin appeared to decrease with age, the changes were not significant and were due to an increased elimination rate in only one calf. There was no change with age in the remaining three calves. Oxytetracycline elimination was significantly reduced in newborn calves. This was exemplified by a decrease in the half-life of elimination t1/2 (beta) from 672.5 +/- 99.4 in the newborn to 385.6 +/- 76.8 at 6 weeks of age, and 377.3 +/- 40.8 min in the 250-day-old calf. These changes were consistent in all four calves. The rate of elimination remained low for the first 4 weeks of life. The volume of distribution Vd, area was not changed after the first week of life. Based on pharmacokinetic changes, an adjustment of dosage is indicated for oxytetracycline in the newborn calf as compared to the older calf or adult.

Newborn calves from dams free of staphyloccocal udder infection were assigned to treatment groups in two experiments. Following colostrum feeding for 2 days, a culture of Staphylococcus aureus was added to pasteurized milk fed to one group twice weekly for a total of nine feedings. A control group received only pasteurized milk. Bull calves were in a short experiment to determine whether the organism was established in body tissues, and a second experiment was to determine the effect on incidence of mastitis at calving. No staphylococcus aureus was isolated from any body tissue or surfaces of bull calves necropsied at about 7 wk of age. Moreover, there was no difference in incidence of staphylococcal udder infection at first calving between heifers exposed to the organism as calves and controls. There appears little reason for concern about detrimental effects of feeding mastitic milk to calves under conditions where they are maintained in individual pens.

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Infusion of each udder quarter at drying-off with 500 mg benzathine cloxacillin, significantly reduced (P < 0.01) the rate of new mastitis infection in cows up to 4 to 10 days post-calving. Rate of new infection was 18.8% in control cows compared to 4.6% in infused cows. 75% of the new infections in cows and 66.7% in quarters were detected only by bacteriological examination. No increase in the less common species of mastitis producing bacteria occurred as a result of dry cow infusion. The incidence of new infections in the period from the first post-calving sampling up to 30 days of lactation was similar in both groups.